1. Field
Exemplary embodiments of the present invention relate to a method, a program and an apparatus for image processing, and more particularly to a method, a program and an apparatus for image processing capable of effectively reducing image noise.
2. Discussion of the Background
When taking pictures with an imaging apparatus in the dark, the duration of exposure is forced to be long. In such a condition, a noise or an isolated noise which is isolated due to dark current, a noise in an electric circuit, a temperature and so forth may easily be generated. In particular, when the duration of exposure is more than 60 seconds, a number of isolated noises are generated. Thus, a noise eliminating method using a shade difference method is frequently used. However, this noise eliminating method has a downside in which a black spot is generated in a saturated pixel. Moreover, the dark exposure is necessary for the same period of time as that of the exposure. When exposing for 180 seconds, an exposure of 360 seconds will be necessary, and it is not realistic. A median filtering method may also be used. However, when exposing for more than 60 seconds, the number of noises significantly increases, thereby increasing demands for applying a strong median filter. The side effect of this is that an edge of the image becomes blurry. Therefore, related arts have proposed techniques for reducing or eliminating isolated noises.
For example, Japanese Patent Laid-Open Application Publication No. 2004-015191 proposes a noise eliminating method in which location information of the noise generated on a CCD is stored in a memory region in advance so that the noise is eliminated using the stored location information of the noise with respect to an acquired image. Japanese Patent Laid-Open Application Publication No. 2003-298952 proposes a noise eliminating method, in which when shooting by a digital camera, a shutter is closed in order to perform an exposure for the same period of time as that of a shooting exposure, and the data of dark current is stored. Using the photographed image and the dark current image, the noise is eliminated. Japanese Patent Laid-Open Application Publication No. 3605084 proposes a noise eliminating method in which a shutter is closed, and the exposed image is subtracted from an image photographed under the same exposure conditions. Japanese Patent Laid-Open Application Publication No. Hei 11-018012 proposes a noise eliminating method using the linearity of four consecutive pixels. In this method, a noise is detected and corrected without storing the noise information.
In the related art, according to Japanese Patent Laid-Open Application Publication No. 2004-015191, the location information of the noise is stored in advance, and noise elimination is performed on the photographed image using the location information of the noise. However, in this noise eliminating method, the location information of the noise needs to be stored. Consequently, the noise may not be eliminated in a short period of time. Furthermore, if the storage condition is the same, a favorable noise elimination result may be obtained. However, if the shooting conditions such as the exposure time and the temperature are changed, the noise conditions may also be changed, thereby making it difficult to accurately eliminate noises.
In the related arts, according to Japanese Patent Laid-Open Application Publication No. 2003-298952 and No. 3605084, a noise is eliminated by storing the dark current data and by subtracting an image equivalent of the dark current from the photographed image. However, in a case where shooting conditions are changed, the dark current may also be changed accordingly. Therefore, it is necessary to store a plurality of image data and to equip with a large memory region. In a case where the image has a saturated pixel, a defective pixel having a black spot is generated, when performing subtraction. Furthermore, in a case of a long exposure time, twice the exposure time may be necessary. Consequently, the noise may not be eliminated in a short period of time, causing a number of problems such as a burden to a user.
In the related arts, according to Japanese Patent Laid-Open Application Publication No. Hei 11-018012, a noise is detected and is corrected using the linearity of four consecutive pixels from the photographed image. However, in a case where the number of noises increases depending on shooting conditions, it may be possible to generate a consecutive isolated noise which is formed of a group of a plurality of pixels. Consequently, the noise may not completely be eliminated. In addition, the processing speed decreases in this method. Therefore, there will be a need for a designated hardware. If there is no designated hardware, there may be a problem in which noise may not be eliminated in a short period of time.